1. Field of the Invention
The invention relates to a playback data detecting device which is suitable for a digital optical disk storage device, hard disk storage device, and digital VTR and, in particular, to a playback data detecting device which is operable to playback data constraining the number of continuous non-inverse bits to at least xe2x80x9c2xe2x80x9d.
2. Description of the Related Art
Generally, in a digital optical disk storage device, hard disk storage device, or digital VTR, a threshold voltage level is determined for discriminating digital playback data. And when a playback voltage level of a bit of the digital playback data exceeds the threshold voltage level, the bit is discriminated as a high level xe2x80x9cHxe2x80x9d, and, otherwise, the bit is discriminated as a low level xe2x80x9cLxe2x80x9d. Such a technique makes use of characteristics of digital recording, and the technique can be done by a simple circuit because a logic for the discrimination is simple. However, whenever the playback voltage level of a bit which is to be discriminated as a low level slightly exceeds the threshold voltage level due to error, a bit error occurs. Also, when once such a bit error occurs, it is not possible to correct the error at a discriminate/playback block.
For this reason, in a part of a digital optical disk apparatus, a detecting method has been adopted which combines a method known as a partial response (1, 1) with three levels and a Viterbi decoding method of two states.
When a signal is recorded on the apparatus, the method converts the signal into a NRZ (non return to zero recording) signal or a NRZI (nonireturn to zero inverted recording) signal using a pre-coder. On the other hand, when playback of the signal is executed, the method detects the signal using the partial response (1, 1). The detecting method by the use of the partial response (1, 1) carries out data detection using the correlation between playback signals. In the method, when a recording signal xe2x80x9c1xe2x80x9d is given, playback signals xe2x80x9c..001100..xe2x80x9d are obtained and level detection of three levels is consequently made.
After the detection due to the partial response (1, 1) is finished, a Viterbi decoding method of three levels and two states is executed. The Viterbi decoding method, taking two playback states, such as state S0 and state S1, makes a transition to state S0 and provides the value of the output signal xe2x80x9c0xe2x80x9d when the signal of xe2x80x9cxe2x88x921xe2x80x9d is input in the state S0. When the signal of xe2x80x9c0xe2x80x9d is input in the state S0, it makes a transition to state S1 and provides the value of the output signal xe2x80x9c1xe2x80x9d. When the signal,of xe2x80x9c0xe2x80x9d is input in the state S1, it makes a transition to state S0 and provides the value of the output signal xe2x80x9c1xe2x80x9d. When the signal of xe2x80x9c1xe2x80x9d is input in the state S1, it makes a transition to state S1 and provides the value of the output signal xe2x80x9c0xe2x80x9d. When the signal violating the above-described state transition rules is inputted, bit error correction is performed by detecting an incorrect state and judging an original state. As a result, the error rate for random errors is improved.
In the above-mentioned method which carries out equalization using the partial response (1, 1) and decoding using the three levels and the Viterbi decoding method of two states, detecting of playback data is performed by bit error correction based on the correlation between the three levels which is provided by the partial response (1, 1). However, when a recording side records data by converting the same into signals which have a minimum inverse metric or interval at least two, namely, the number of continuous non-inverse bits equal to at least xe2x80x9c2xe2x80x9d, the feature of the signals may not be fully utilized. More specifically, it is to be noted that a bit sequence includes at least two continuous signals of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d and, as a result, non-inverse bits lasts for at least two bits.
Then, the present inventor has already proposed, in Japanese Laying-Open Publication No. H06-267203(namely, 267203/1994), a playback data detecting device which is operable to perform bit error correction by making use of the above-mentioned feature of the signals. The device is used to record the signals, such as (1, 7) signals which has the continuous non-inverse bits of at least xe2x80x9c2xe2x80x9d. The decoding method used in the device, taking four playback states as state S0 through state S3. In this event, the decoding method makes a transition to state S0 and provides the value of the output signal xe2x80x9c0xe2x80x9d when the signal of xe2x80x9cxe2x88x921xe2x80x9d is input in the state S0. Likewise, when the signal of xe2x80x9c0xe2x80x9d is input in the state S0, it makes a transition to state S1 and provides the value of the output signal xe2x80x9c1xe2x80x9d. When the signal of xe2x80x9c1xe2x80x9d is input in the state S1, it makes a transition to state S2 and provides the value of the output signal xe2x80x9c0xe2x80x9d. When the signal of xe2x80x9c1xe2x80x9d is input in the state S2, it makes a transition to state S2 and provides the value of the output signal xe2x80x9c0xe2x80x9d. When the signal of xe2x80x9c0xe2x80x9d is input in the state S2, it makes a transition to state S3 and provides the value of the output signal xe2x80x9c1xe2x80x9d. We the signal xe2x80x9c0xe2x80x9d. xe2x80x9cxe2x88x921xe2x80x9d is input in the state S3, it makes a transition to state S0 and provides the value of the output signal xe2x80x9c0xe2x80x9d. When the signal violating the above-described state transition rules is inputted, bit error correction is performed by detecting an incorrect state and judging an original state. As a result, the error rate for random errors is improved. Thus, it might be said that the above-mentioned method is equivalent to a combination of the equalization of partial response (1, 1)+a Viterbi decoding method of three levels and four states.
With the playback data device using the combination of the equalization of partial response (1, 1)+the Viterbi decoding method of three levels and four states, the feature of the signals can be almost used. However, the feature is not still used completely, because only three levels of information are utilized about information included in the playback signals. Therefore, the device can be further improved.
Then, the present inventor further proposes, in Japanese Laying-Open Publication No. H05-334811 (namely, 334811/1993), the playback data detecting device which can make use of information in the playback signals more effectively. The device is also used to record signals which have the number of continuous non-inverse bits equal to at least two. The decoding method used in the device produces playback. And the decoding method, taking four playback states such as state S0 through state S3, makes a transition to state S0 and provides the value of the output signal xe2x80x9c0xe2x80x9d when the three levels of xe2x80x9cxe2x88x921xe2x80x9d and the two levels of xe2x80x9cxe2x88x921xe2x80x9d are input in the state S0. When the three levels of xe2x80x9c0xe2x80x9d and the two levels of xe2x80x9c1xe2x80x9d are input in the state S0, this makes a transition to state S1 and provides the value of the output signal xe2x80x9c1xe2x80x9d. When the three levels of xe2x80x9c1xe2x80x9d and the two levels of xe2x80x9c1xe2x80x9d are input in the state S1, it makes a transition to state S2 and provides the value of the output signal xe2x80x9c0xe2x80x9d. When the three levels of xe2x80x9c1xe2x80x9d and the two levels of xe2x80x9c1xe2x80x9d are input in the state S2, it makes a transition to state S2 and provides the value of the output signal xe2x80x9c0xe2x80x9d. When the three levels of xe2x80x9c0xe2x80x9d and the two levels of xe2x80x9cxe2x88x921xe2x80x9d are input in the state S2, it makes a transition to state S3 and provides the value of the output signal xe2x80x9c1xe2x80x9d. When the three levels of xe2x80x9cxe2x88x921xe2x80x9d and two levels of xe2x80x9cxe2x88x921xe2x80x9d are input in the state S3, it makes a transition to state S0 and provides the value of the output signal xe2x80x9c0xe2x80x9d. When the signal violating the above-described state transition rules is inputted, bit error correction is performed by detecting an incorrect state and judging an original state. As a result, the error rate for random errors is improved. Therefore, the method is a combination of equalization of partial response (1, 1) with a Viterbi decoding method of three levels/two levels and four states.
In the above-mentioned decoding method which is realized by a combination of equalization of partial response (1, 1)+the Viterbi decoding method of three levels/two levels and four states, an amount of information to be obtained is twice as large as an amount of information which is obtained from the method using only the three levels and the method using only the two levels. Therefore, in the method, the feature of the playback signals is more effectively used as compared to the method which is a combination of equalization of partial response (1, 1)+the Viterbi decoding method of three levels and four states. Therefore, the ability to correct bit error can be further improved.
However, an information processing device has been developed to become high-performance and miniaturization of the device is more sophisticated. According to development of the device, a demand to higher recording density and higher speed recording/playback is growing from year to year. Therefore, it is highly expected that the above-mentioned playback data detecting device is developed to create a device which has excellent ability to correct bit errors.
Therefore, it is an object of the invention to provide a playback data detecting device which has higher ability to correct bit errors by improving the above described device which employs the equalization of partial response (1, 1) together with the Viterbi decoding method of three levels/two levels and four states.
Also, it is an object of the invention to provide a playback data detecting device which has still higher ability to correct bit errors by extending the equalization of partial response (1, 1)+the Viterbi decoding method of three levels/two levels and four states to Viterbi decoding method of six states or Viterbi decoding method of ten states.
According to a first aspect of the invention, there is provided a playback data detecting device which includes a PR(1, 1) equalizer which receives playback signals of the recording data, converts the signals into three levels of data and two levels of data which resides in the intermediate point between the three levels of data in time by PR(1, 1) detecting which employs the correlation between the signals, and a Viterbi decoder of four states which performs Viterbi decoding based on transition rules for transition of playback states S0, S1, S2, and S3, which correspond to the three levels of data and the two levels of data.
According to a second aspect of the invention, there is provided a playback data detecting device which includes a PR(1, 2, 1) equalizer which receives playback signals of the recording data, converts the signals into four levels of data and seven levels of data which resides in the intermediate point between the four levels of data in time by PR(1, 2, 1) detecting which employs the correlation between the signals, and a Viterbi decoder of six states which performs Viterbi decoding based on transition rules for transition of playback states S0, S1, S2, S3, S4 and S5, which correspond to the four levels of data and the seven levels of data.
According to a third aspect of the invention, there is provided a playback data detecting device which includes a PR(1, 2, 2, 1) equalizer which receives playback signals of the recording data, converts the signals into seven levels of data and ten levels of data which resides in the intermediate point between the seven levels of data in time by PR(1, 2, 2, 1) detecting which employs the correlation between the signals, and a Viterbi decoder which of ten states performs Viterbi decoding based on transition rules for transition of playback states S0, S1, S2, S3, S4, S5, S6, S7, S8 and S9, which correspond to the seven levels of data and the ten levels of data.